Preparation And Properties Of Co/Mn-Based Catalysts For Oxygen Reduction Reaction

Fuel cells are considered as the most clean and efficient energy conversion technology in the future. But due to its sluggish kinetics of the oxygen reduction reaction at the cathode, electrocatalyst is necessary. The most efficient and mostly used catalyst by far is Pt-based materials. However, as the high cost and scarcity of Pt-based catalysts have hindered the widespread application and commercialization of fuel cells, the research on non-precious metal catalysts is of high significance. Transition metal chalcogenides have attracted interest for its low cost and well catalytic activity for oxygen reduction reduction. Among its conventional synthesis methods, hydrothermal/solvothermal method is very simple, and the morphology and size of catalysts can be well controlled by surfactant-assisted synthesis.In this thesis, MnO2nanorods and carbon supported CoSx nanoparticles had been prepared through hydrothermal and solvothermal method respectively, and CoMn composite catalyst had been synthesized from hydrotalcite precursors. Varieties of analytical tools had been carried out for the characterization of crystal structure and morphology and the effect of the amount of surfactant added, calcining, and particle size on the catalytic performance had been studied. The results are as follows:(1) MnO2nanorods prepared with0.01MCTAC showed an ORR onset potential at about0.89V, half-wave potential at2500rpm was0.67V and the peak current was reached at about0.7V. After18000s of continuous operation in0.1M KOH, the stability of MnO2is about15%higher than Pt/C.(2) CoSx/C nanoparticles prepared with0.03A/CTAB and heat-treating showed an ORR open-circut potential at about1.0V and the half-wave potential at2500rpm was0.725V. Over the potential range of0.35-0.70V, the measured HO2-yields were below2%, giving an electron transfer number of ca.3.98, indicating a four-electron pathway. After18000s of continuous operation in0.1M KOH, the stability of CoSx/C is about15%higher than Pt/C. Considering the effects of particle size on ORR performance, sample with an average particle size of2.5nm showed the highest activity.(3) CoMn composite catalysts synthesized from hydrotalcite precursors showed an ORR onset potential at0.84V with high limited-diffusion current density and a nearly four-electron reduction of O2.Sample prepared from CoMnAl-NO3-LDH precursor reacted with KMnO4and heat-treated showed a current density of about5.1mA/cm2at0.2V (2500rpm) and gave an electron transfer number close to4over the potential range of0.2～0.6V, suggesting nearly four-electron reduction of O2in this potential region.